This invention relates generally to the polishing and planarization of semiconductor substrates and, more particularly, to the conditioning of polishing pads in slurry-type polishers.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes successively less planar. This occurs because the distance between the outer surface and the underlying substrate is greatest in regions of the substrate where the least etching has occurred, and least in regions where the greatest etching has occurred. With a single patterned underlying layer, this non-planar surface comprises a series of peaks and valleys wherein the distance between the highest peak and the lowest valley may be the order of 7000 to 10,000 Angstroms. With multiple patterned underlying layers, the height difference between the peaks and valleys becomes even more severe, and can reach several microns.
This non-planar outer surface presents a problem for the integrated circuit manufacturer. If the outer surface is non-planar, then photolithographic techniques to pattern photoresist layers might not be suitable, as a non-planar surface can prevent proper focusing of the photolithography apparatus. Therefore, there is a need to periodically planarize the substrate surface to provide a planar surface. Planarization, in effect, polishes away a non-planar, outer surface, whether a conductive, semiconductive, or insulative layer, to form a relatively flat, smooth surface. Typically, an insulative layer is deposited across the entire surface to be planarized filling valleys but also covering peaks in the surface. Planarization thus removes this layer from above the peaks leaving a substantially uniform planar surface. Following planarization, additional layers may be deposited on the outer layer to form interconnect lines between features, or the outer layer may be etched to form vias to lower features.
Chemical mechanical polishing is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head, with the surface of the substrate to be polished exposed. The substrate is then placed against a rotating polishing pad. The carrier head may also rotate and/or oscillate to provide additional motion between the substrate and polishing surface. Further, a polishing slurry, including an abrasive and at least one chemically-reactive agent, may be spread on the polishing pad to provide an abrasive chemical solution at the interface between the pad and substrate.
Important factors in the chemical mechanical polishing process are: the planarity of the substrate surface, uniformity, and the polishing rate. Inadequate planarity can produce substrate defects. The polishing rate sets the time needed to polish a layer. Thus, it sets the maximum throughput of the polishing apparatus.
Each polishing pad provides a surface which, in combination with the specific slurry mixture, can provide specific polishing characteristics. Thus, for any material being polished, the pad and slurry combination is theoretically capable of providing a specified planarity on the polished surface. The pad and slurry combination can provide planarity in a specified polishing time. Additional factors, such as the relative speed between the substrate and the pad, and the force pressing the substrate against the pad, affect the polishing rate and planarity.
Because inadequate planarity can create defective substrates, the selection of a polishing pad and slurry combination is usually dictated by the required planarity. Given these constraints, the polishing time needed to achieve the required planarity sets the maximum throughput of the polishing apparatus.
It is important to take appropriate steps to counteract any deteriorative factors which either present the possibility of damaging the substrate (such as by scratches resulting from accumulated debris in the pad) or reduce polishing speed and efficiency (such as results from glazing of the pad surface after extensive use). The problems associated with scratching the substrate surface are self-evident. The more general pad deterioration both decreases polishing efficiency, which therefore increases cost, and creates difficulties in maintaining consistent operation from substrate to substrate as the pad decays.
The glazing phenomenon is a complex combination of contamination and thermal, chemical and mechanical damage to the pad material. When the polisher is in operation, the pad is subject to compression, shear and friction producing heat and wear. Slurry, including the abraded material from the wafer and pad, is pressed into the pores of the pad material and the material itself becomes matted and even partially fused, all of which reduce the pad's ability to apply fresh slurry to the substrate.
It is, therefore, desirable to continually condition the pad by removing trapped slurry, and unmatting or re-expanding the pad material.
A number of conditioning procedures and apparatus have been developed. Common are mechanical methods wherein an abrasive material is placed in contact with the moving polishing pad. For example, a diamond coated screen or bar which scrapes and abrades the pad surface to a moderate extent both removes the contaminated slurry trapped in the pad pores and expands and re-roughens the pad. With such systems, abrasive particles from the conditioner may themselves become dislodged from their source and will become contaminates for the pad and the slurry. Further, the mechanical grinding away of the pad reduces pad life. The mechanical abrasive elements themselves are also quite expensive, typically comprising embedded diamond particles, and their use imposes the further downtime required to break-in the abrasive. Typically, a new abrasive element must be broken-in by running it on a pad for approximately thirty minutes to remove any loose abrasive particles prior to the polishing of any wafers so as to avoid scratching the wafers.
An alternative method which largely avoids the dangers of contamination is the ultrasonic agitation of the slurry as disclosed in U.S. Pat. No. 5,245,796 of Gabriel L. Miller and Eric R. Wagner, issued Sep. 21, 1993 (hereinafter Miller, et al.). Miller, et al. discloses the use of an ultrasonic generator placed one-half inch above the pad surface and oscillated at a frequency of 40 KHz to dislodge grit and debris which become embedded in the pad. Miller, et al., however, fails to address the mechanical deterioration of the pad that occurs with glazing.
It is, accordingly, desirable that a conditioner remove debris from the pad and undo glazing while avoiding the introduction of additional mechanical abrasive to the slurry, thus restoring the mechanical structure of the pad without doing unwanted amounts or types of mechanical damage to the pad.